Proposed is a 5 year prospective study to assess the impact of needle exchange programs (NEPs) and other AIDS interventions among injection drug users (IDUs) in natural settings. The study incorporates a pre-test post- ttest control group design with qualitative and ethnographic methods. As suggested in NIDA Program Announcement #94-010, the proposed project builds upon and extends existing cohorts, in this case two cohorts launched in 1992 in the communities of West Oakland and Richmond, CA. The study has three principal research aims; (1) To measure the impact of NEPs at the individual and community levels, and to examine other strategies for access to unused injection equipment; (2) To document needle use practices among IDUs in natural settings, and to specify situational and environmental factors related to drug injection hygiene and multiperson use of equipment; (3) To determine the prevalence and incidence of HIV in each study community and the prevalence of HIV in syringes exchanged at NEPs. Cohorts will consist of 250 IDUs in each community and be observed at six-month intervals, for a total of 1,000 interviews and serum samples per annum. Cohort design will be open, allowing for replacement of cohort members who drop out and making possible the measurement of prevalence as well as incidence. Cohort participation includes risk assessment interview, phlebotomy for HIV testing of serum, HIV results pre- and post-test counseling and referral. The qualitative component proposes to follow 30 active iDUs for purposes of documenting the evolution of drug injection practices in response to changes in public policies, HIV prevention program, and other developments over the 5 year life of this study. The ethnographic methods to be used include participant observation, qualitative interviews and video-taped observation of actual injection episodes in natural settings. The study design allows for analysis of communities before, during and after NEP implementation; for comparison of communities with and without NEPs; for comparison of NEP users with non-NEP users; and for comparison of various levels of NEP use on behavioral and serological outcomes.